34 THOMAS STEEKY HUNT ON A NATUIUL SYSTEM IN 



volume of about 402 were assigned, would be 32(MA), the two polysilicates being 

 respectively, in the atomic notation adopted-, (si.aaL^CaJO^, and (sijsalj.NaJO^. Petalite 

 having the volume of these, and its composition not being then definitely settled, was 

 referred to the same general formula, while orthoclase, from its less density, was conjec- 

 tured to be •)0(M2Oo). As regards the homœomorphous tricliuic feldspars it was then said 

 that " between anorthite and albite may be placed vosgite, labradorite, audesine, and oligo- 

 clase, whose composition and densities are such that they all enter into the same general 

 formula with them, and ha^-e the same equivalent volume. The results of their analyses 

 are by no means constant, and it is probable that many, if not all of them may be variable 

 mixtures of albite and anorthite. Such crystalline mixtures are very common ; thus in 

 the alums, aluminium, iron and chromium, and potassium and ammonium, may replace 

 one another in indefinite proportions. . . . Heintz has shown by fractional precipi- 

 tation that there are mixtures of homologous fatty acids, which cannot be separated by 

 crystallization, and have hitherto been regarded as distinct acids. The author insists that 

 the possibility of such mixtures of related species should be constantly kept in view in 

 the study of mineral chemistry. The small portions of lime and potash in many albites, 

 and of soda in anorthite, petalite, and orthoclase, are to be ascribed to mixtures of other 

 feldspar-species." 



§ 24. These conclusions were reiterated in 1855, in a paper giving the results of my 

 chemical studies of these feldspars (when Scheerer's hypothesis was noticed), and it was 

 said that similar A'iews "must also be extended to the scapolites.'" Some years later, in 

 1864, Tschermak- put forth a view similar to that advocated by Von Waltershausen and 

 myself, and maintained that the feldspars proper were reducible to three species, adularia 

 or orthoclase, albite and anorthite. While recognizing the fact that certain potash-soda feld- 

 spars (such as perthite) are made up of alternations of orthoclase and albite, he further 

 concluded, as I had already done, " that oligoclase, audesine and labradorite appear to be 

 members of a great series with many transitional forms, and may be regarded as isomor- 

 phous mixtures of albite with anorthite, sometimes with small admixtures of orthoclase." 



§ 25. With regard to this conception of the nature of these intermediate feldspars, it 

 should be noted that the chemical difficulties in the way of verifying it are much greater 

 than in the case of soluble compounds, where, as in the case of the fatty acids just men- 

 tioned, solution and separation by fractional precipitation are possible, or where differ- 

 ences in A'olatility may be appealed to. While a definite feldspar-species having the com- 

 position assigned to labradorite doiibtless exists in nature, it is, nevertheless, true that a 

 mixture of proportions of anorthite and albite containing equal parts of alumina, would 

 give a centesimal composition identical with that assigned to labradorite, just as the com- 

 position of a fatty acid may be simulated by a mixture of its higher and lower homo- 

 logues. In so far as the view of Von Waltershausen and myself, since adopted by Tscher- 

 mak, is true, the action of acids capable of attacking the basic feldspars will enable irs to 

 discriminate between admixtures and definite intermediate species. That the latter should 

 occur in nature is, a priori, probable from the composition of the parallel series of the 



' Examinations of some Feldspathic Bocks ; L. E. & D. Philos. Mag., May, 1855. 



■ Tschermak, 1864, K. K. Académie Wissenschaft, Wion, and Pojzg. Ann, ]865, v, 139, .See .ilso t.Iio antlior's 

 C'licin. and Geol. Essays, p. 444. 



